1. Field of the Invention
The invention relates to contact devices in electrical units, especially contact devices which are provided on the circuit boards of small electrical appliances, for example watches, cameras and electric shavers.
2. Description of the Prior Art
Such small appliances are generally equipped with their own power source. It is known to use circuit boards in such appliances to control the proper supply of current. The circuit boards can consist, for example, of hard paper and preferably have copper strips as conductors, which are expanded at various points into so-called soldering lugs. The soldering lugs are used to receive the feed lines from electronic components, for example capacitors or transistors, or the feed lines of contact devices which interact with the circuit board.
By soldering the feed lines from the electronic components with the conductor strips of the circuit board, the components are connected electrically with the conductor strips and mechanically with the circuit board. This is generally done using machine-type soldering methods, for example the flow-solder method or the drag-solder method. Besides conductor strips, resistor strips of various resistances may also be printed on such circuit boards. Such printed-on resistors, whose resistance depends on the material utilized and on the dimensions of the resistor strip, can be applied, for example, with so-called resistor pastes in a screen printing method.
The circuit board can also have contact surfaces which are part of an electrical contact device and which are contacted by a movable element, for example a spring plate or a contact slide. Such contact surfaces are generally gilded, to prevent an impairment of the conductivity of the contact surface by corrosion, and thus guarantee a reliable contact over the longest possible time. However, silver, nickel, or tinned contact surfaces have also been used for this purpose. Such circuit boards are quite expensive to fabricate, because the various layers must be applied in several electroplating baths, while covering the remaining circuit board surface. Apart from this, the expense of the nobel metal layers is quite considerable.
With such circuit boards, it is also necessary that the contact surfaces, which interact with other parts of the contact device, be completely covered by suitable means during the machine-type soldering process of the circuit board, so as to prevent contamination of the contact surfaces by solder which may adhere from the soldering bath. All these process steps make fabrication of a circuit board very expensive if this circuit board has both soldering lugs for receiving electronic components as well as contact surfaces associated with a contact device.
Another disadvantage of the known contact devices, which have contact surfaces affixed to circuit boards, consists in the fact that, again to avoid corrosion, the remaining contact surfaces of the contact device, which are not affixed on conductor strips, must be provided with conducting and corrosion-resistant casings. Application of these casings can also take place in electroplating baths, where the above-mentioned metals are again used.
In principle, all contact surfaces must therefore be provided with corrosion-resistant casings, regardless whether or not they contain contact surfaces that are affixed on circuit boards. This is true, for example, for spring-plate contact devices, which do not have contact surfaces affixed on conductor strips, but whose terminals are soldered to the circuit board, so as to establish an electrical or mechanical connection with the latter. It is also true for contact devices which are not connected with a circuit board either electrically or mechanically.
Another common disadvantage of all known contact devices, whose contact surfaces touch one another and thereby slide against one another, is the necessity of subsequently greasing the metal-coated contact surfaces. Otherwise, due to corrosion, operational disturbances could occur, which would make necessary the complete replacement of the contact device.